1) Field of the Invention
The present invention relates to an electroluminescent (EL) display apparatus in which self-luminescent elements such as organic light emitting diodes (OLEDs) and thin film transistors (TFTs) for driving the self-luminescent elements are arranged in a matrix, and the driving method thereof, and more specifically, relates to a voltage-write type EL display apparatus in which nonuniform luminance does not occur even in a large screen display apparatus, and the driving method thereof.
2) Description of the Related Art
The organic EL display apparatus using an OLED is recently attracting attention because of a wide angle of visibility, high contrast, and excellent visibility, as compared with a liquid crystal display apparatus using a liquid crystal device. Since the organic EL display apparatus does not require a backlight, a thin and light display can be realized, and hence it is also advantageous in view of power consumption. Further, the organic EL display apparatus has features such that the response speed is fast since direct current low-voltage driving is possible, it is strong against vibrations since the display apparatus is formed of solid, it has a wide operating temperature limit, and a flexible shape is possible.
A conventional organic EL display apparatus will be explained below, mainly about an active matrix panel. FIG. 13 indicates the active matrix panel and a driving circuit in the schematic configuration of the conventional organic EL display apparatus. In FIG. 13, in the active matrix panel 100, display cells 110 are arranged at each point of intersection of n scan lines Y1 to Yn and m data lines X1 to Xm, and the basic structure is similar to that of the active matrix type liquid crystal display apparatus.
The active matrix panel 100 includes, as the liquid crystal display apparatus, a scan line driving circuit 120 that supplies a scan line select voltage at a predetermined timing with respect to the n scan lines Y1 to Yn and a data line driving circuit 130 that supplies a data voltage at a predetermined timing with respect to the m data lines X1 to Xm. In FIG. 13, other types of circuit for driving the organic EL display apparatus are omitted.
In the active matrix panel 100, the point different from the liquid crystal display apparatus is that the respective display cells 110 include the OLED instead of the liquid crystal device. As the configuration of the display cell 110, a so-called voltage write type display cell is well known, which includes a select TFT, a drive TFT, a capacitor, and an OLED one each (for example, see Japanese Patent Application Laid-open Publication No. H8-234683, hereinafter, “first patent document”).
One example of an equivalent circuit in the voltage write type display cell is such that, as shown in FIG. 13, the gate of the select TFT is connected to the scan line and the drain to the data line, and the gate of the drive TFT is connected to the source of the select TFT, and the source to a common line (in many cases, a ground line GND). The capacitor is connected between the source and gate of the drive TFT, and the anode side of the OLED is connected to a supply voltage line (Vdd in the figure), with the cathode side thereof connected to the drain of the drive TFT.
The operation of the voltage write type display cell will be explained briefly. When the scan line select voltage is supplied from the scan line driving circuit 120 to the gate of the select TFT, the select TFT becomes the ON state, so that the data voltage supplied from the data line driving circuit 130 is applied to the gate of the drive TFT and the capacitor. As a result, the drive TFT becomes the ON state, and a current path from the cathode side of the OLED to the common line is formed. In other words, the OLED emits light by the current determined corresponding to the data voltage. On the other hand, the data voltage is stored in the capacitor.
The stored data voltage is supplied to the gate of the drive TFT due to the connection between the drive TFT and the capacitor. Therefore, even when the scan line select voltage is not supplied to the gate of the select TFT, that is, after the scan line driving circuit 120 has shifted to the selection of the next scan line, the OLED continues to emit light until the next scan line is selected by the scan line driving circuit 120. In other words, the OLED continues to emit light by the data voltage written in the capacitor. Hence, this type of display cell is referred to as the voltage write type.
The first patent document relates to the voltage write type organic EL display apparatus, and other than this, a current write type organic EL display apparatus that can solve the problem of nonuniform luminance described later has also been proposed (for example, see Japanese Patent Application Laid-open Publication No. 2001-147659 hereinafter, “second patent document”).
However, the organic EL display apparatus adopting the voltage write type display cell has a problem in that nonuniform luminance occurs in realizing a large screen. It is known that this problem occurs because the properties of the drive TFT (for example, threshold voltage Vth) are different between the display cells, even on a normal-size screen. Various solutions with respect to the problem due to the difference in the drive TFT have been proposed, and hence further explanation is omitted here.
The occurrence of nonuniform luminance due to a large screen is not attributable to the difference in the drive TFT, but attributable to wiring resistance of the common line. This problem will be explained below. FIG. 14A illustrates a display cell line of the i-th line in the active matrix panel 100. As shown in FIG. 14A, in m display cells on the i-th line, the sources of the respective drive TFTs are all connected to the same common line 31. In other words, while all drive TFTs are in the ON state, the currents i1 to im flowing to the respective OLEDs flow to the same common line 31. The common line 31 is formed of a highly conductive material, but has wiring resistance more or less (resistance R1 to Rm+1 in the figure), and when the length thereof becomes long with an increase of the screen size, a voltage drop due to the wiring resistance cannot be ignored.
Normally, since high definition is realized with an increase of the screen size, the number of the display cells in the line direction also increases. This means that the sum total of the current flowing into the common line 31 increases, which causes a further increase in the voltage drop due to the wiring resistance. Therefore, when the luminance of the active matrix panel 100 is made the highest, the current value flowing into the common line 31 becomes the largest. FIG. 14B explains a voltage drop in the common line. The common lines 31 are arranged, as shown in FIG. 13, for each line, and in parallel with the line direction, and the opposite terminals thereof are connected to a common power source. Since the common power source is a grounded potential in many cases, the current flowing into the common line 31 from the respective display cells is divided by a current value corresponding to the inflow position and directed to the opposite terminals of the common line 31. Therefore, when the wiring length of the common line 31 is designated as L, as shown in FIG. 14B, the potential at a position of 0.5L from one end of the common line 31 becomes maximum, taking into consideration that the wiring resistance is superimposed according to the position from the end of the common line 31. The maximum value Vmax is expressed by the following equations:                               V          max                =                                            1              2                        ·            r            ·            i            ·                                          (                                                      m                    +                    1                                    2                                )                            2                                ⁢                                           ⁢                      [                          m              ⁢                              :                             ⁢              odd              ⁢                                                           ⁢              number                        ]                                                                                         V          max                =                              1            2                    ·          r          ·          i          ·                      m            2                    ·                                    (                                                m                  +                  2                                2                            )                        ⁢                                                   [                          m              ⁢                              :                            ⁢                                                           ⁢              even              ⁢                                                           ⁢              number                        ]                                                           where the current flowing to the respective OLEDs is designated as “i”, and a resistance of the wiring resistance of the common line 31 corresponding to between the display cells is designated as “r”.
In the organic EL display apparatus, since all OLEDs are made to emit light steadily, the current flows from the respective display cells to the common line 31, even immediately before writing a new data voltage in the capacitor in the display cell. In other words, even immediately before writing a data voltage, the potential of the common line 31 has a size corresponding to the position of the display cell in which the data voltage is written, that is, a size according to the potential distribution as shown in FIG. 14B. As seen from the configuration of the display cell shown in FIG. 14A, since one terminal of the capacitor is connected to the common line 31, the voltage written in the capacitor has a size based on the potential of the common line 31. In other words, even when the data having the same voltage value is input respectively to the display cells on the first row and the display cells on the m/2-th row, the voltage written in the capacitor in the respective display cells is different.
For example, even when a data voltage Vsig is supplied to all data lines Xi to Xm from the data line driving circuit 130, the voltage Vsig is written in the capacitor in the display cell located on the data line Xi in FIGS. 14A and 14B, and a voltage Vsig−Vmax which is smaller than the voltage Vsig is written in the capacitor in the display cell located on the data line X0.5L. That is, the active matrix panel 100 becomes dark in the central portion, and brighter towards the edges. This is an important problem in realizing a large size and high luminance in the active matrix panel 100.
The second patent document discloses a current write type display cell, but in this current write type, it is necessary to provide a minute current of a precise value to the respective display cells. With an increase of the screen size, the current control becomes difficult. Further, the current write type display cell requires more (for example, four) TFTs than being required in the voltage write type display cell, in order to form the display cell, this causes problems in improving a numerical aperture of the display cell and in cost reduction.